The present invention is directed to a surround audio compatibility assessment method, system, and apparatus, and more particularly to a surround audio compatibility assessment method, system, and apparatus that is or is associated with an audio monitor.
“Monophonic sound” (also referred to as “mono”) is the reproduction of an audio source (sound) using a single audio channel that is often centered in the sound field (analogous to a visual field). “Stereophonic sound” (also referred to as “stereo”) is the reproduction of an audio source using independent audio channels through a symmetrical configuration of speakers. The term “stereo” is almost exclusively used to describe two-channel (left and right) sound, although technically more than two channels could be used. “Surround sound” (also referred to as “surround”) encompasses a range of techniques for reproduction of an audio source with audio channels reproduced using multiple discrete speakers. A surround sound system creates the illusion of multi-directional sound through speaker placement and signal processing. Surround sound is characterized by a listener location or sweet spot where the audio effects work best, and presents a fixed or forward perspective of the sound field to the listener at this location.
Most modern motion pictures and prime-time television shows (referred to jointly as “media content”) are produced in surround. Being the premier audio format, mixing engineers understandably put their attention on how their content sounds in surround. Though most theaters will reproduce the media content in surround, the eventual release on DVD for the home market will not experience the same uniformity of presentation. Indeed, as is the case with sound for digital television, the majority of viewers of movie DVDs will experience the audio in stereo and a nontrivial percentage will hear it in mono.
The conversion of surround to stereo or of stereo to mono involves combining channels and algebraically summing their waveforms. Signals that are present in multiple channels may cancel, or partially cancel, when those channels are combined. The degree of cancellation depends on their relative phase, the ratio of their levels prior to combining and any level adjustment introduced in the process of combining. If the original signals have equal amplitudes and are of opposite phase the signal will be completely absent from the combination. The more insidious situation occurs, however, when just one component in a surround mix appears in multiple channels but shifted in phase. This can easily happen when a single source is picked up by multiple non-coincident microphones. When the outputs of these microphones are combined, there will be cancellations and the signal level will be reduced. If this happens to an actor's voice, the dialog can become unintelligible.
Mono compatibility of stereo material has traditionally been monitored with a Lissajous display. The Left and Right channels drive the vertical and horizontal channels of an oscilloscope. Equipment specifically designed for audio monitoring (e.g. a sound “monitoring product” or “audio monitor”) typically will rotate the display counterclockwise by 45 degrees to make the left channel appear as a diagonal line tilting toward the upper Left and the Right channel appear as a line tilting toward the upper right. Interpretation of such a display requires experience associating the various shapes with circumstances in which audio has experienced cancellations when mixed to mono.
Many manufacturers have eliminated the graphical display in their sound “monitoring product” or “audio monitor” by using “correlation” meters. These correlation meters multiply the Left and Right channels together and average the result, creating an indicator that is positive when the channels are in-phase and negative when they are out-of-phase. This is usually normalized by the channel levels, creating an indicator scaled between +1 and −1. A good stereo signal will hover near zero, a good mono signal will be positive. Indications that go very negative represent problem content that will cancel when reproduced in mono.
Surround sound “monitoring products” or “audio monitors” also use Lissajous or correlation displays. The first problem in monitoring surround audio compatibility with either type of display is the sheer number of channel pairs involved. Ignoring the LFE (Low Frequency Effects) channel, a 5.1 surround program (e.g. Dolby Digital and DTS (Digital Theater System)) contains 10 channel pairs. A 6.1 surround program has 15 channel pairs. A 7.1 surround program has 21 channel pairs. FIG. 1 shows five speakers 100 each interconnected with channel pairs (e.g. neighboring channel pairs 102 and LF/RF channel pair 104 where “LF” is the left front speaker and “RF” is the right front speaker). This is the five main channels of a 5.1 surround program. The LFE is not shown in this figure. The 6.1 and 7.1 surround programs would have a similar pattern in which arrows connect all channel pairs, but the resulting diagram would be extremely busy. Many commercial surround sound monitoring products only analyze neighboring channel pairs that are shown in FIG. 1 as the outside double arrows 102. Other commercial surround sound monitoring products add the LF/RF channel pair 104.
The challenge for the user is watching numerous correlation meters or Lissajous patterns simultaneously. Vendors of such tools have used various schemes to pack these displays onto a single XY display. All of these schemes take advantage of the redundancy evident in the four quadrants of the Lissajous display. Since the lower half of a Lissajous display offers no additional information compared to the upper half, the display may be truncated or folded at the horizontal axis.
Monitoring audio signals through a broadcast chain has long been a job for humans, skilled in audio, well versed in the potential problems and attentively listening to the program on an accurate reproduction system. Particularly in television broadcast, such people are scarce. The recent explosion of television channels and delivery systems has drastically increased the number of programs to be monitored. The shift to surround sound has added additional failure mechanisms such as front/rear channel reversal and compatibility with stereo and mono reproduction. Economic realities have further constrained both the availability of skilled personnel and the acoustic quality of their monitoring environment while reducing the time available to accomplish the task.
The issues facing professionals and organizations creating and delivering surround programs include, but are not limited to: mixing and monitoring surround is a far more complex and challenging task than it is for stereo programs as there are many more opportunities for error; budgets, both financial and time, are shrinking; personnel are expensive and skilled personnel are very expensive; people get tired and bored so when things don't go wrong often (hopefully), vigilance is difficult to maintain; and record keeping is important for post-mortem analysis and for assessing financial accountability, but people hate to keep records.